1. Field of the Invention
The present invention relates to a blank for a ring member of a bearing, which is used in manufacturing a ring member (inner race or outer race) of a bearing provided to a rotational portion of a precision apparatus or the like and a manufacturing method for the same. Also, the present invention relates to a manufacturing method for a ring member of a bearing and a bearing which is manufactured using the manufacturing method.
2. Description of the Related Art
Up to now, there have been increasing strong demands for an entire downsizing and attaining a higher density of a hard disk drive. For this reason, with regard to a ball bearing, too, which is indispensable for the hard disk drive, such as a pivot bearing, there has been demanded a compact and high accuracy product.
FIG. 13 is a cross-sectional view showing a structural example of a ball bearing. A ball bearing 1 shown in the figure includes a plurality of balls (spherical bodies) 4 sandwiched between an inner race 3 and an outer race 2. In the following description, the inner race 3 and the outer race 2 are generally called “ring members.”
Conventional ring members to be used in precision apparatuses such as a hard disk drive are formed using a columnar blank. First, in order to subject the columnar blank to turning processing, the columnar blank is gripped by a lathe chuck. Then, a solid center portion of the columnar blank is subjected to drill processing by a drill. With regard to the blank now having a substantially-cylindrical shape, which is formed by the drill processing, the cutting processing is performed on an inner circumferential surface and an outer circumferential thereof, to thereby form a desired ring shape. In this case, in the cutting processing of the outer race and the inner race, cutting processing resistance in the drill processing by a drill is much larger compared with that in the cutting processing of the inner circumferential surface and the outer circumferential surface. Therefore, a gripping force of the chuck should be increased conforming with the drill processing.
However, along with downsizing of the bearing, the ring member is made thinner. Accordingly, the cylindrical member is liable to deform by receiving the gripping force of the chuck. For example, in a case where the chuck has a three-claw structure, the cylindrical shape deforms into a triangular shape, whereas in a case where the chuck has a six-claw structure, the cylindrical shape deforms into a hexagonal shape. Those deformations may be observed using a roundness measuring device. For that reason, in a case where the turning processing is performed on the above-mentioned columnar blank to obtain the ring shape, the gripping force of a chuck should be adjusted while paying a careful attention. If the gripping force carelessly increases, roundness of the ring member decreases. Further, if the gripping force of the chuck reduces, the columnar blank detaches in the drill processing generating a large processing resistance, and the production thereof is suspended, causing problems.
Meanwhile, in order to improve yields by reducing materials to be wasted, a technique is disclosed in which, as shown in FIG. 14A, a formation material 5 of a bearing preform is formed by die forging. The formation material 5 is formed such that three cylindrical portions R1, R2, and R3 each having different diameters and being race preforms of the bearings are formed with steps in an axial direction, and a base portion 6 is provided inside the cylindrical portion R3 having the smallest diameter (refer to, for example, JP Patent Publication No. 2000-167641, referred to hereafter as Patent Document 1).
By the way, according to the related art described in Patent Document 1, as shown in FIG. 14B, the formation material 5 formed by die forging is formed such that stepped portions coupling the cylindrical portions each having the different diameters are sequentially punched by a separation punch, and that the cylindrical portions R1, R2, and R3 each having the different diameters are separately and independently formed. For this reason, as a countermeasure for obtaining corrosion resistance or the like required for a bearing of a hard disk device, in a case where stainless steel is used as a material thereof, in an inner circumferential surface of the cylindrical portion R1 separated to be independent from a formation material 5′, there exist an upper fluidized bed portion 7 formed by forging, and a lower (having height h) separated layer portion 8 formed by punching. Similarly, in an outer circumferential surface of the formation material 5′, there exist the lower fluidized bed portion 7 formed by forging, and the upper (having height h) separated layer portion 8 formed by punching.
It should be noted that an entire outer circumferential surface of the cylindrical portion R1 is formed by die forging, and thus the entire surface becomes the fluidized bed portion 7.
In other words, when the die forging is performed with respect to stainless steel as a material, the surface formed in contact with the die becomes the fluidized bed portion 7, and has, compared to the separated layer portion 8 having the height h separately formed by punching the fluidized bed portion 7, different surface characteristics including higher hardness. This is because, on the surface of the fluidized bed portion 7, a crystal grain size decreases, which is different from that of the separated layer portion 8, and the eutectic carbides are oriented in one direction.
The difference of the surface characteristics adversely affects the finishing cutting processing with respect to the inner circumferential surface and the outer circumferential surface of the cylindrical portion R1, such as cutting load applied to a bite. Thus, it is difficult to process the entire cylindrical surface of a bearing in uniform with high-accuracy. Therefore, even in the case of forging stainless steel as a material, there is desired, by providing the entire cut target surface with the uniform surface characteristics, a blank for a ring member of a bearing enabling highly accurate cutting processing. Besides, in removing processing such as cutting processing or grinding, fallen crystal grains deteriorate processing accuracy. Accordingly, there is desired a manufacturing method for a blank for a ring member of a bearing having a reduced crystal grain size.